1. Field of the Invention
This invention relates to an improved gauge for registering temperature, oil pressure, fuel level, and the like which has a case designed to eliminate or greatly reduce the fogging of the sight glass, viewing lens or window therein.
2. Description of Related Art Including Information Disclosed Under 37 CFR SS1.97-1.99.
As is well known, gauges used in vehicles, airplanes, and in industry are prone to fogging depending upon atmospheric conditions, especially when the sight glass or window lies in a horizontal plane. If the sight glass or window becomes cooler than the interior of the gauge, moisture within the gauge will deposit on the inside of the window. This can be a serious problem if the fogging effect renders the gauge unreadable.
Although this invention is not intended to be limited to electrically operated gauges, in a preferred embodiment the gauge movement constitutes a coil which is coupled to a remote sender. The sender is a variable resistance which controls the current flow through the coil, which in turn causes deflection of the needle. The needle rests on a bobbin which includes a magnet disposed within the coil. The sender or variable resistance could measure, as will be obvious to those skilled in the art, a wide variety of variables such as temperature, fuel level, oil pressure, or it could measure voltage or amperage. The sender then controls a flow of current to the movement within the gauge.
The flow of current through the gauge movement coil is a heat generator. Accordingly, the interior of the gauge and the sight glass window will be heated slightly. If the external temperature cools the sight glass window, the window can fog from the inside, depending upon atmospheric conditions. Such prior art gauges and senders are available commercially from, for example, the assignee of this invention, Datcon Instrument Company, for example, Series VIII, gauges, senders and meters.
It has been proposed in the past to use a double window having spaced-apart glass or plastic lenses in order to minimize fogging. See, for example, U.S. Pat. No. 2,701,969 wherein the space between the windows communicates with the external atmosphere, and U.S. Pat. No. 1,925,999 wherein a partial vacuum is created between the windows. In addition, in U.S. Pat. No. 3,530,723, an inner plastic bubble is used to isolate the gauge mechanism from a glass cover. In that patent, however, the space between the bubble and the glass cover, which functions as a sight window, is sealed.
While it is desirable from the standpoint of reducing the tendency to fog to provide two sight windows which function similar to a storm window, it is not desirable to seal the space between the two. If the seal is not perfect, or if the gauge is subjected to extreme atmospheric conditions, it can leak. If moisture enters the space between the two sight windows, it will then be very difficult to remove without totally dismantling the gauge. For this reason, the patentee in U.S. Pat. No. 3,530,723 uses a desiccant material in the sealed space. Furthermore, it is not desirable to have the space communicating with the ambient atmosphere as described in U.S. Pat. No. 2,701,969 because, in many environments, moisture will invade the space and fog the glass when it condenses, for example, on the upper surface of the inner lens.